Creating RAIDs via GNOME Disks, won’t be possible in version 3.12.
Another cool feature seems that will soon land in Gnome. This time Gnome makes “RAID Creation” easy, a work of Red Hatter David Zeuthen. David (G+, blog) mostly hacks gnome-disk-utility and together with the UX team is responsible for the new options and design in Gnome Disks.
Red Hater David Zeuthen in his blog introduces all the new features in Gnome Disks 3.6 and also refers to the upcoming new options for Gnome 3.8. There are some nice additions; some are just optics and some are useful tools, but in my opinion not everything is perfect and complete.